This section provides background information related to the present disclosure which is not necessarily prior art.
In known transmissions of this kind, the transmission lubrication and transmission cooling are typically adapted to particularly high speeds of rotation and/or for particularly high temperatures. Known transmissions usually have optimum efficiency at an operating temperature in the range of 70° C. to 90° C. since the liquid lubricant, usually a lubrication oil, has sufficiently low viscosity with simultaneously sufficiently good lubrication capability in this temperature range and the liquid lubricant as well as also other components of the transmission are characterized by a particularly good service life in this temperature range. At transmission temperatures below the optimum operating temperature range, the transmission efficiency is reduced due to increased churning losses which result from a higher viscosity of the liquid lubricant.
As is known, the operating temperature of a motor vehicle transmission is influenced by head wind cooling. The heat dissipation achieved by the head wind cooling is thus dependent on the vehicle speed and on the external temperature and is configured in practice to high external temperatures and to a simultaneously high power introduction into the transmission, e.g. at low driving speeds such as on mountain passes or on desert trips.
It has proven problematic with known transmissions which are used in motor vehicles that the optimum operating temperature range is as a rule only reached after some journey time. In this respect, the journey time required to achieve the optimum operating temperature range is typically considerably longer than the duration of test cycles for the standardized determination of the fuel consumption and/or of the pollutant emission. The determination of the fuel consumption or of the pollutant emission in other words does not usually take place in the optimum operating state of the transmission. It can furthermore also occur in practice that, with a slower manner of driving which is more favorable for consumption and/or at particularly low external temperatures, the optimum operating temperature range, and thus the optimum transmission efficiency, are only reached very late if at all.